EP1496379A2 - Fibre et lentille de Fresnel - Google Patents

Fibre et lentille de Fresnel Download PDF

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Publication number
EP1496379A2
EP1496379A2 EP04254121A EP04254121A EP1496379A2 EP 1496379 A2 EP1496379 A2 EP 1496379A2 EP 04254121 A EP04254121 A EP 04254121A EP 04254121 A EP04254121 A EP 04254121A EP 1496379 A2 EP1496379 A2 EP 1496379A2
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EP
European Patent Office
Prior art keywords
zone
refractive index
optical device
optic axis
discontinuities
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP04254121A
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German (de)
English (en)
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EP1496379A3 (fr
Inventor
Michael Charles Parker
Stuart Douglas Walker
Makiko Hisatomi
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Fujitsu Ltd
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Fujitsu Ltd
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Filing date
Publication date
Application filed by Fujitsu Ltd filed Critical Fujitsu Ltd
Publication of EP1496379A2 publication Critical patent/EP1496379A2/fr
Publication of EP1496379A3 publication Critical patent/EP1496379A3/fr
Withdrawn legal-status Critical Current

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
    • G02B6/03688—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 5 or more layers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/023—Microstructured optical fibre having different index layers arranged around the core for guiding light by reflection, i.e. 1D crystal, e.g. omniguide
    • G02B6/02304—Core having lower refractive index than cladding, e.g. air filled, hollow core
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
    • G02B6/03605—Highest refractive index not on central axis
    • G02B6/03611—Highest index adjacent to central axis region, e.g. annular core, coaxial ring, centreline depression affecting waveguiding
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/30—Optical coupling means for use between fibre and thin-film device
    • G02B6/305—Optical coupling means for use between fibre and thin-film device and having an integrated mode-size expanding section, e.g. tapered waveguide
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02385—Comprising liquid, e.g. fluid filled holes

Definitions

  • the present invention relates to a microstructure for an optic fibre and a similar structure for a lens.
  • Optic fibres are used to guide light over metres to many kilometres. They work by confining the light to a central core. Examples of known structures for optic fibres are briefly described below.
  • the core that carries the light is surrounded by a layer of cladding, as may be seen in Figure 1a.
  • Both the core and cladding are typically glass but the core is made of a high refractive index material whilst the cladding has a lower refractive index, so that light in the core undergoes total internal reflection at the interface between the two layers and so is confined to the core.
  • Figure 1b is an example of the refractive index variation.
  • photonic waveguiding structures such as the so-called "OmniGuide” fibre (see Figure 2), which has been described at M. Ibanescu et al, Physical Review E, Vol . 67(4), article number 046608, 2003.
  • alternate concentric layers of high and low refractive index surround a core, the structure being periodic in the radius.
  • the periodicity acts as a bandgap to exclude certain wavelengths and thus confine them to the core.
  • Figure 4 shows one proposal to overcome this. Instead of simply abutting the end of, in this example a polymer waveguide to the end of a silicon waveguide, a tapered section of the silicon waveguide is extended into the polymer core over some distance. The taper collects more of the light from the polymer core and transmits it into the silicon core.
  • the device is further described at T Shoji et al , 15th LEOS Con f. Proc., Vol. 1, TuU3, p289-290, 2002.
  • Another proposal is to use a short section of graded index fibre as a lens to concentrate light from the wide exit aperture of the fibre or polymer waveguide into the smaller aperture at the end of the silicon waveguide.
  • the action of such a lens is illustrated in Figure 5, which shows the rays from the wide glass optic fibre aperture being converged by the index variation to a focus, which is where the input aperture to the silicon waveguide is placed.
  • the present invention provides a new structure for confining light to the core of an optic fibre.
  • the structure is also useful for making a lens.
  • Figure 6a shows one particular example of a zoned microstructured fibre lens according to the present invention.
  • the lens comprises a number of concentric cylinders, or "zones" of optical material.
  • zones the refractive index is graded from a value of 2.5 at its inner radius to a value of 2.1 at its outer radius.
  • the gradation follows a segment of a parabola.
  • the refractive index changes between 2.1 and 2.5 discontinuously. (As will become clear these two values, and some further parameters as well, can be changed.)
  • n m ( r ) n 1 - ( n 1 - n ' m ( r ))( n 1 - n 2 ) N n 1 ⁇
  • n m (r) is the refractive index variation within the m th zone (with n 1 being the upper value at each discontinuity and n 2 being the lower)
  • r is the radius of a particular point
  • N is the total number of zones
  • r N is the outer radius of the outermost zone.
  • this structure is reminiscent of the Fresnel zone plate, which of course is a plate having a number of concentric zones.
  • the zoned structure of the invention has a focussing property, as does the Fresnel zone plate, and also the Fresnel zone plate has a square root variation for the radii of its zones, which this particular example of the invention having the parabolic refractive index variation does as well (see equation 2). Based on these similarities the inventors have decided to call the zoned microstructure of the invention a Fresnel zone microstructure.
  • the present invention is not a Fresnel zone plate; the Fresnel zone plate diffracts light through its apertures and then the light propagates through free space to its focus, whereas in the present invention light is refracted continuously as it passes through the material of the device.
  • Equation 3 is the general equation of a geometric light ray trajectory (i.e. light considered as a particle) through a medium in accordance with the "principle of least time".
  • n( x ) is the refractive index of the medium at point x in the medium
  • t is the (scalar) trajectory of the ray.
  • Both sets of rays show paths oscillating about the axis of the device. This oscillation confines light propagating along the device to a region about the axis and so the structure may be used for an optic fibre, i.e. waveguide. (The fibre waveguide using the structure would of course be much longer than the lens shown in Figure 6a.)
  • the device will bring rays injected parallel to the axis to a focus on the axis after one quarter of a period. Therefore a section of the structure a quarter of the period long will make a lens having a focal length equal to that length. A use of this, to couple light between optical devices, is given later.
  • Figure 7b shows the calculated ray paths for such lens having the same overall diameter as the zoned example and the refractive index varying form 3.5 as the axis to 1 at the outside, which provides similar focussing power to the zoned example.
  • These ray paths are also sinusoids but rays injected on-axis have a substantially longer period than those injected off-axis and so the focus for rays injected over a wide aperture will be poor.
  • Figures 7c and 7d show (being enlargements of portions of Figures 7a and 7b), the focus for a set of rays injected at the same point but over a range of injection angles is better for the zoned device. Again a tighter focus for the zoned lens is evident and is useful because not all of the light incident on the lens will be at the same angle.
  • zoned device whether for use as a lens or an optic fibre waveguide
  • the refractive index needs only to be varied over a narrow range (2.1 to 2.5 is used in this example) compared to the unzoned case (in the comparative unzoned example above the range was 1 to 3.5).
  • Layered optic fibres are usually constructed by assembling a large preform of glass (or other material) of cylinders of the appropriate refractive indexes, and the preform is then drawn into a fibre. If the refractive indexes are very different then very different materials will have to be used, which may well differ in their mechanical and thermal properties, which will make this process of construction difficult.
  • a lens can be made by drawing a fibre and then taking a short section of that.
  • the section can be in length just a single quarter period of the sinusoidal oscillation but an odd integer multiple of that would also work and may be useful if it is difficult to obtain such a short length or if the devices coupled by the lens are an inappropriate distance apart.
  • n 1 and n 2 the maximum and minimum refractive indexes
  • the total number of zones and the total radius of the fibre although considering equations 1 and 2 these are not all independent.
  • the period of the sinusoids is not key for the design of an optic fibre, although probably if it is too long it will limit the ability to bend the fibre without loss of the light.
  • the focal length (and hence the period of the sinusoids) is usually of interest.
  • Figure 8 shows a possible lens application.
  • a glass single mode fibre (SMF) has typically a core of 9 ⁇ m in diameter, whereas a photonic crystal waveguide is, for example, manufactured from silicon; this has a much higher refractive index, and so to be single mode will have a waveguide width of 0.3 ⁇ m.
  • SMF glass single mode fibre
  • a microstructured zoned lens placed between the two will focus light exiting the broad aperture of the single mode fibre into the smaller aperture of the photonic crystal waveguide and in particular a lens comprised of a quarter period section of the zoned structure (or an odd integer multiple of that length) works well for the reasons noted above, namely the focussing of rays injecting at different r and the good focusing of rays injected at different angles. Further as explained below such a lens provides impedance power matching, i.e. minimal Fresnel reflection back into the single mode fibre (or back into the photonic crystal depending on the direction of operation).
  • the focal length is important because it relates the input and output spot sizes.
  • f the focal length
  • ⁇ the wavelength of the light
  • n 1 the refractive index as above
  • a a measure of the width of the pattern of light incident on the lens
  • 2w o the width of the aperture of the photonic crystal waveguide.
  • the focal length i.e. quarter of the sinusoid period
  • n 1 and n 2 i.e. the refractive index contrast
  • n N the number of zones
  • the equivalent simple unzoned graded index rod lens described above has a wide range of refractive index but for this purpose the value can be taken roughly to be that at the axis (where the light is concentrated), namely 3.5, which is far from the ideal value and significant reflection into the single mode fibre results. It is difficult, however, to make this figure lower because the refractive index at the edge of the fibre is already low, i.e. 1.
  • FIG. 9a A second example of a zoned optical structure according to the invention is shown in Figures 9a and 9b.
  • the structure comprises a number of concentric zones with a discontinuity in refractive index between each zone and a continuous variation of that within each zone.
  • the refractive index at the beginning of all zones is the same and the refractive index at the end of all zones is the same.
  • the refractive index in each zone is a segment of a Gaussian function (again centred on the axis, i.e the form is exp(-Kr 2 ), where r is the radius from the optic axis).
  • n m ( r ) n 1 exp(- ⁇ 2 m r 2 /2) + ⁇ n m
  • r is the radius from the optic axis
  • n 1 is the refractive index on the optic axis
  • ⁇ m is a constant for each zone determined as explained below
  • ⁇ n m is a constant for each zone chosen to make the refractive index equal to n 1 at the beginning of each zone.
  • Equation 7 shows that perfect sinusoid ray trajectories are possible.
  • Equation 7 shows that perfect sinusoid ray trajectories are possible.
  • Substituting the parabolic zone structure defined by equations 1, 1a and 1b (for the first zone) into the ray trajectory equation 3a results in: d 2 r dz 2 ⁇ -2( n 1 - n 2 ) N n m ( r ) r 2 N r ⁇ -2 ⁇ N r 2 N r
  • n m (r) is not quite a constant but is not far off because zoning leads to a low refractive index contrast (i.e. n 2 ⁇ n 1 ) and so again sinusoids result.
  • the examples of the zoned microstructures given were for parabolic and Gaussian refractive index variation.
  • the Taylor expansion of the Gaussian has terms in r 2 (quadratic i.e. parabolic), r 4 , r 6 and so on, so there will be functions having, or having Taylor expansions having, the quadratic plus certain amounts of the higher order terms that will produce sinusoidal trajectories.
  • These functions can be zoned using the procedures described above. Further examples of possible functions are the hyperbolic tangent function (tanh) and the hyperbolic secant function (sech).
  • zoning of continuous refractive index profiles gives the advantage that it reduces the refractive index contrast and so makes them easier to fabricate. Zoning will nonetheless maintain further properties of interest - e.g. interesting non-linear optical properties.
  • the examples have been of refractive index functions that decrease monotonically within the zones, which is useful to refract rays back towards the axis. Using increasing functions would appear to direct rays away from the axis, which may be useful, for example, for constructing a diverging lens. Equally although the examples have shown the value of the refractive index on the axis to be high this is not essential.
  • the refractive index curve for each zone can be reduced in width or extended arbitrarily (its slope or ⁇ having been determined) and the jump in refractive index at the zone boundary can also be determined arbitrarily. Adhering to the rule of constant upper and lower refractive indexes across the zones, however, keeps the overall refractive index contrast low, and is easier to fabricate.
  • these structures can also be approximated to give broadly similar results. For example once the zone radii are determined for a particular profile (parabolic etc.) the gradation of refractive index in each zone could be approximated by a linear segment. However this may not be much easier to fabricate than the parabolic (etc.) variation itself.
  • Another approximation would be to have the refractive index constant within a zone (again the zone radii are determined using the parabolic (etc.) variation), but changing to a different value at the zone boundary.
  • that would be one particular value for odd zones and another for even zones.
  • the central zone would have the higher value, but it could have the lower value.
  • An exemplary binary refractive index profile of a fibre with a higher refractive index in the central zone is given in Figure 11. In particular this has zone boundaries according to equation (2).
  • FIG. 12 An exemplary binary refractive index profile of a fibre with a lower refractive index in the central zone is given in Figure 12, which have the inverse refractive index profile of the fibre shown in Figure 11, i.e. it has the lower refractive index n 2 where Figure 11 has the higher n 1 and vice versa.
  • a particular type of fibre with a lower refractive index in the central zone is a hollow core fibre.
  • Figures 13 and 14 show example zonal refractive index profiles of hollow core fibres with a binary refractive index profile in zones outside the central zone. These fibres employ refractive index profiles comparable to that of Figures 11 and 12 respectively, in the outer zones.
  • the core of this type of fibre may be filled (as is known in the art) with air; providing a reduced attenuation compared to solid core fibres due to reduced material absorption losses.
  • the core of hollow core fibres can also be filled with a microfluid, to enable the fibre to be used as sensor.
  • the guiding properties of the fibre for example absorption, degree of evanescence, modal field diameter
  • the behaviour of the sensor can be defined by the material used to fill the central zone of the fibre.
  • Figure 15 shows a further possible refractive index profile of a hollow core fibre.
  • the zones outside the hollow core in this profile are those shown in Figure 6b.
  • Figure 16 shows the refractive index of a fibre where the refractive index profile of the outer zones is an inverted version of that of the fibre shown in Figure 15. Again an exponential variation as opposed to the parabolic is possible.
  • Figure 17 shows a binary hollow core fibre consisting of concentric rings of voids and glass zones.
  • the voids may be filled with air or fluids as discussed above.
  • support sections are required at intervals along the core to support the structure. Provided these support structures are small they do not significantly affect propagation of light along the fibre.
  • the inverse structure where the central zone has a refractive index of n 2 with the outer zones alternating between n 1 and n 2 (n 1 in the second zone) is possible.
  • the zones then comprise planes of optical material at distances from a central plane (as opposed to cylinders of material around a central axis as described above).

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Optical Couplings Of Light Guides (AREA)
  • Optical Integrated Circuits (AREA)
  • Optical Fibers, Optical Fiber Cores, And Optical Fiber Bundles (AREA)
EP04254121A 2003-07-11 2004-07-09 Fibre et lentille de Fresnel Withdrawn EP1496379A3 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0316306A GB2403816A (en) 2003-07-11 2003-07-11 Optical device with radial discontinuities in the refractive index
GB0316306 2003-07-11

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EP1496379A2 true EP1496379A2 (fr) 2005-01-12
EP1496379A3 EP1496379A3 (fr) 2005-07-27

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EP (1) EP1496379A3 (fr)
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GB (1) GB2403816A (fr)

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JP4451696B2 (ja) * 2004-03-30 2010-04-14 富士通株式会社 微細構造ファイバの非線形係数の波長依存性をキャンセルする装置
US20080285927A1 (en) * 2006-04-24 2008-11-20 Sterlite Optical Technologies Ltd. Single Mode Optical Fiber Having Reduced Macrobending and Attenuation Loss and Method for Manufacturing the Same
US8315526B2 (en) * 2007-06-18 2012-11-20 Hewlett-Packard Development Company, L.P. Misalignment tolerant free space optical transceiver
US7724439B2 (en) * 2007-10-24 2010-05-25 Aptina Imaging Corporation Lens, a lens array and imaging device and system having a lens, and method of forming the same
CN102539015B (zh) * 2012-02-15 2013-11-20 长飞光纤光缆有限公司 一种分布式温度传感光纤
US8824848B1 (en) * 2013-06-10 2014-09-02 Sumitomo Electric Industries, Ltd. Multimode optical fiber including a core and a cladding
CN103487876B (zh) * 2013-09-09 2016-02-03 曲阜师范大学 一种用于3-5微米波段光波宽带低损耗传输的空芯光子带隙光纤
US9442246B2 (en) * 2013-10-14 2016-09-13 Futurewei Technologies, Inc. System and method for optical fiber
US10310146B2 (en) 2014-06-09 2019-06-04 Vadient Optics, Llc Nanocomposite gradient refractive-index Fresnel optical-element
WO2017058146A1 (fr) * 2015-09-28 2017-04-06 Vadients Optics Llc Élément optique de fresnel nanocomposite à gradient d'indice de réfraction

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US5243618A (en) * 1991-11-22 1993-09-07 Hughes Aircraft Company Cavity resonator incorporating waveguide filter
US5673353A (en) * 1993-03-02 1997-09-30 Ward; Robert M. Fiber and lens preform with deep radial gradient layer and method of manufacturing same
US20020001445A1 (en) * 2000-05-15 2002-01-03 Sumitomo Electric Industries, Ltd. Optical fiber
US20020094181A1 (en) * 1999-03-17 2002-07-18 Bhagavatula Venkata A. Dispersion shifted optical waveguide fiber
WO2002057820A1 (fr) * 2001-01-19 2002-07-25 The University Of Sydney Fibre optique
US20020191929A1 (en) * 1998-10-14 2002-12-19 Massachusetts Institute Of Technology, A Massachusetts Corporation Omnidirectional multilayer device for enhanced optical waveguiding
US20030031407A1 (en) * 2001-01-31 2003-02-13 Ori Weisberg Electromagnetic mode conversion in photonic crystal multimode waveguides
WO2003038494A1 (fr) * 2001-10-30 2003-05-08 The University Of Sydney Guide d'onde optique exempt de diffraction
US6573813B1 (en) * 1999-04-23 2003-06-03 Massachusetts Institute Of Technology All-dielectric coaxial waveguide with annular sections
WO2003050571A2 (fr) * 2001-12-11 2003-06-19 Blazephotonics Limited Procede et appareil portant sur les guides d'ondes en fibres optiques

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US5243618A (en) * 1991-11-22 1993-09-07 Hughes Aircraft Company Cavity resonator incorporating waveguide filter
US5673353A (en) * 1993-03-02 1997-09-30 Ward; Robert M. Fiber and lens preform with deep radial gradient layer and method of manufacturing same
US20020191929A1 (en) * 1998-10-14 2002-12-19 Massachusetts Institute Of Technology, A Massachusetts Corporation Omnidirectional multilayer device for enhanced optical waveguiding
US20020094181A1 (en) * 1999-03-17 2002-07-18 Bhagavatula Venkata A. Dispersion shifted optical waveguide fiber
US6573813B1 (en) * 1999-04-23 2003-06-03 Massachusetts Institute Of Technology All-dielectric coaxial waveguide with annular sections
US20020001445A1 (en) * 2000-05-15 2002-01-03 Sumitomo Electric Industries, Ltd. Optical fiber
WO2002057820A1 (fr) * 2001-01-19 2002-07-25 The University Of Sydney Fibre optique
US20030031407A1 (en) * 2001-01-31 2003-02-13 Ori Weisberg Electromagnetic mode conversion in photonic crystal multimode waveguides
WO2003038494A1 (fr) * 2001-10-30 2003-05-08 The University Of Sydney Guide d'onde optique exempt de diffraction
WO2003050571A2 (fr) * 2001-12-11 2003-06-19 Blazephotonics Limited Procede et appareil portant sur les guides d'ondes en fibres optiques

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Also Published As

Publication number Publication date
US7110649B2 (en) 2006-09-19
EP1496379A3 (fr) 2005-07-27
US20050031262A1 (en) 2005-02-10
JP2005031687A (ja) 2005-02-03
GB2403816A (en) 2005-01-12
GB0316306D0 (en) 2003-08-13

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